The following U.S. patents are believed to represent the current state of the art:
U.S. Pat. Nos. 3,945,469; 4,018,306; 4,037,685; 4,042,066; 4,406,351; 4,424,884; 4,469,198; 4,531,611; 4,538,704; 4,569,418; 4,650,036; 4,664,226; 4,830,141; 4,865,155; 4,919,228; 5,065,839; 5,127,491; 5,377,778; 5,392,877; 5,497,855; 5,620,058 and 6,318,503.